Examples of communications systems that require synchronous communications include Code Division Multiple Access (CDMA) networks, such as those compliant with 3GPP and 3GPP2 specifications, and WiMAX (i.e., IEEE §802.16) compliant networks. CDMA technology, for example, allows multiple cellular phone users to share the same frequency spectrum, and uses a generated noise carrier with a different and essentially orthogonal instance of the noise carrier assigned to each mobile unit within a cell. The base station receiver in a CDMA station correlates the received signal from a mobile unit with the desired noise carrier, extracting the transmitted digital signal with a sufficient signal-to-noise ratio to achieve a satisfactory data error rate. Because the base stations in a system such as CDMA must be synchronized with surrounding base stations to handle handoff of mobile phones between cells and for other functions, a time reference must be provided to each base station for clock synchronization. This is commonly provided through Global Positioning System (GPS) receivers which comprise a part of each base station. It is envisioned that any satellite based time reference systems, such as the Global Navigation Satellite System (GLONASS), Galileo Satellite System, or any other satellite based system could also be used for this purpose. In one embodiment, GPS satellites each provide radio signals that are synchronized and usable by GPS receivers to derive a very accurate time reference. GPS receiver antennas of cellular phone equipment are often placed high relative to surrounding terrain to obtain the best coverage area, and as a result may be subject to lightning damage and other physical damage from installation and handling. Such damage or other operating disruptions may cause CDMA base stations to lose contact with GPS satellites, and the period of time during which the base station loses the signal of the GPS system is called the holdover period. The communication system should ideally continue to operate during this holdover period until contact can be reestablished, whether through repair of damaged equipment or other service.
During holdover, base stations may rely on a local clock reference. For example, a crystal oscillator may provide a time reference during this holdover period, as long as the oscillator is stable enough to keep the base station sufficiently synchronized with other base stations. When no GPS signal is received, the base station operates in holdover mode, and the oscillator signal is generated by a crystal oscillator designed to provide a signal of the same frequency as is provided by the GPS receiver. If the oscillator is not sufficiently stable, the time it provides to the base station may drift with respect to the desired GPS reference time, and cause the base station to fail to communicate properly. Attempts to use a packet switched network (PSN), such as an internet protocol (IP) network, to correct for oscillator drift have been unsuccessful because of the inherent nature of PSN networks, which are difficult to model. IP network packets travel through various routes between the same source and destination depending upon network conditions, making it difficult to predict what route a particular packet will take, and therefore, its transit time. Furthermore, the amount of time required for packet transfer may vary depending upon dynamic network traffic conditions. Consequently, conventional time stamp based time synchronization over PSN suffer from the inherent variations and network delays that are a byproduct of packet switched networks. Thus, an ongoing need exists for systems and methods for improving on the performance of current CDMA base station oscillator stability when a base station loses the lock to the GPS signal and operates in holdover mode. As the occurrence of a loss of GPS signal lock is typically not a planned event, it is desirable to have a timing synchronization scheme that can be readily implemented at any time. The present systems and methods disclosed herein address this need through the use of time independent packet data.